1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to an image forming apparatus of electrophotographic type using a contact charging method, such as a copier or a printer.
2. Description of the Related Arts
Heretofore, a corona discharger has often been used as means for charging a photoreceptor of a copier or printer of an electrophotographic type. However, a contact charging method has recently been used in which a charger having a shape like a brush, roller, blade, or belt is allowed to be in contact with the photoreceptor or is held opposite to the photoreceptor with a small gap maintained therebetween.
This method can obviate a defect, when a device using a corona discharger generates poisonous ozone or a high voltage of about several kilovolts must be applied to the corona discharger for providing a charging potential required for the photoreceptor.
Further, the contact charging method may involve a problem that the contact state between the charger and the photoreceptor is not uniform and it is not possible to obtain a uniform surface potential because there is a variation in the surface shape and surface roughness of the photoreceptor or in the shape, physical properties, or the like of the contact charger. Accordingly, various attempts have been made to overcome this problem. For example, Japanese Unexamined Patent Application No. SHO 63(1988)-149669 proposes a method in which a DC current with a superposed AC current is applied to the charger.
However, this method is accompanied by a drawback a charging irregularity (unevenness) called strobing, banding, or cycle spots is generated in the moving direction of the photoreceptor due to the AC voltage component. If the AC voltage is high, the surface potential will be high; whereas if the AC voltage is low, the surface voltage will be low. In the reverse developing method, which is often used in a laser printer or the like, the developed density will be low when a portion with a high surface potential is exposed to light, whereas the developed density will be high when a portion with a low surface potential is exposed to light.
In the meantime, the frequency of the AC voltage component is suitably selected in accordance with the electrophotographic process used in the device. Typically, a frequency within a range of several tens Hz to several hundreds Hz is used. If the exposure frequency in the sub-scanning direction is near the frequency of the AC voltage component, there arises a case in which a portion having a high surface potential is exposed to light and a case in which a portion having a low surface potential is exposed to light. When black is recorded on the entire surface of the recording paper, a larger density portion and a smaller density portion will appear alternately and a fringe pattern corresponding to the frequency of the AC voltage component is observed. Also, when nothing is recorded, a density unevenness of ground fogging will appear.
Sometimes, these phenomena may not be observed if the frequency of the AC voltage component is high. However, if an image is to be formed on a lateral line, which is a main scanning direction, alternately in white/black lines, or in one white line/two black lines, or in two white lines/one white line, a fringe pattern appears due to an interference with the charging unevenness generated with an interval corresponding to the frequency of the AC voltage component, making the record extremely difficult to discern.
FIG. 5 shows a state of a photoreceptor surface potential in the case where an image is to be formed in alternate black line/white line in a state uniformly charged with the photoreceptor surface potential of 700V. The portion (hatched portion) being lower than the developing bias potential (400V) is developed in black. When an unevenness of photoreceptor surface potential is generated by the AC voltage component applied to the charger, the developed image of a line will be narrow and thin when a high potential portion is exposed to light as shown in FIG. 6(b), whereas it will be wide and thick when a low potential portion is exposed to light as shown in FIG. 6(a).
Further, in the case where the frequency of the AC voltage component does not coincide with the pitch and period of the exposure to light, both a line developed to be wide and thick and a line developed to be narrow and thin will appear on one page of the recording sheet. This is caused by the interference between the period of the charging unevenness and the period of exposure to light. The interference is observed as a fringe pattern with thick/thin portions having a period which is equal to the difference of the periods.
Thus, when the frequency of the charging unevenness caused by the AC voltage component and the frequency of the scanning lines is slightly different, the difference generates what is called a "beat" in acoustics, thereby producing a fringe pattern.
For example, if characters are printed at 600 dpi in an apparatus with a photoreceptor velocity of Vp=25 mm/s and an AC voltage component of f=300 Hz, the unevenness of the surface potential is generated on the photoreceptor at a pitch of Vp/f=0.08333 mm. In the case where a black line and a white line are alternately printed, the pitch of black scanning lines will be 25.4/600.times.2=0.04233.times.2.apprxeq.0.084667 mm. Since the frequency of the black scanning lines is 25/0.084667.apprxeq.295.2755 Hz. Thus difference in frequency is 300 Hz-295.2755 Hz=4.7245 Hz. Thus a fringe pattern with thick/thin portions having a period of 25/4.7245=5.29 mm appears on the photoreceptor surface, and the image quality is extremely deteriorated.
Methods for improving this situation are disclosed in U.S. Pat. No. 4,727,453 and No. 4,851,960. U.S. Pat. No. 4,727,453 discloses that a contact charger is brought into contact with a photoreceptor for a time interval of 1/f provided that the frequency of the AC voltage is f. In this method, the frequency of the AC voltage, the surface velocity of the photoreceptor, and the contact width must be maintained in a predetermined relationship, so that the construction of the apparatus will be constrained. On the other hand, U.S. Pat. No. 4,851,960 has the following constraints. Namely, the amplitude of the AC voltage component must be more than twice the initial charging voltage, and the shape of the charger must be such that it gradually departs away from the photoreceptor surface.
Also, it sometimes happens that, if the frequency of the AC voltage component approaches the spatial frequency represented by lateral lines on a recording sheet having an image formed thereon, a fringe pattern is generated in the image. In order to prevent this, Japanese Examined Patent Publication No. HEI 07(1995)-89249 discloses that the frequency f of the AC voltage component and the moving velocity Vp of the photoreceptor are adjusted so that N times or 1/N times the minimum interval of the scanning lines (where N is a natural number) does not overlap the variation range of the spatial wavelength (Vp/f).
Here, the fringe pattern in question is generated as follows. When a portion of the charging roller having a weakened endurance has an electrostatic capacitance, the charging phase is shifted to produce cycle spots which are curved in the axial direction of the photoreceptor. If a lateral line coincides with this curved portion having a low potential, it is developed thickly to generate the fringe pattern. Therefore, the method disclosed in the above Publication does not prevent generation of the fringe pattern when the frequency of the charging unevenness caused by the AC voltage component and the frequency of the scanning lines is slightly different.
Even without using the above-mentioned methods, it is possible to avoid this phenomenon by separately adding an oscillator which generates a frequency of an AC voltage component and adjusting the frequency so as to allow the frequency to completely coincide with the scanning pitch. However, this solution increases the costs of the apparatus and necessitates an apparatus-by-apparatus adjustment.